PLASMA CUTTING METHOD AND NC PLASMA CUTTING APPARATUS

Abstract: The present invention relates to a plasma cutting method and a NC plasma cutting apparatus in which an improvement can be made in the quality of a bore-cut product obtained by using a transferred plasma arc, and the productive efficiency can also be enhanced. In order to achieve the above-described advantages, the transferred plasma arc is switched to a non-transferred plasma arc in a position immediately before a cutting finishing point and in the vicinity of a position in which cutting lines cross each other. Simultaneously with this switching operation, the moving speed of a plasma torch is varied in accordance with a change in the plasma arc current. Additionally, the transferred plasma arc current is reduced in a position immediately before the cutting finishing point and in the vicinity of a position in which cutting lines cross each other. The moving speed of the plasma torch may be varied in accordance with the reduction in the arc current. Further, the above-described voltage detecting function is kept in the off state during a cutting operation, and it is actuated in a position immediately before the cutting finishing point and in the vicinity of a position in which cutting lines cross each other.

Description: TECHNICAL FIELD The present invention relates to a plasma cutting method and a NC plasma cutting apparatus. More particularly, the invention relates to a plasma cutting method and a NC plasma cutting apparatus best suited to cutting a workpiece by generating a transferred plasma arc. BACKGROUND ART Hitherto, a plasma cutting method has been widely employed as a processing method for cutting plate workpieces, such as iron, stainless steel, aluminium and the like, at high speed. Plasma cutting apparatuses used for this method are classified into a transferred type and a non-transferred type depending on the method of generating a high-temperature plasma arc. A typical non-transferred plasma cutting apparatus, as schematically shown in Fig. 6, comprises an electrode 11 disposed at the center of a plasma torch and a funnel-shaped nozzle 12 which is placed to surround the forward end of the electrode 11 across a plasma gas passage and which has an opening at the forward end thereof. Both the electrode 11 and the nozzle 12 are connected to the output terminal of a power supply source 16. With this construction, during a cutting operation, arc discharge is generated between the electrode 11 and the nozzle 12 while a plasma gas is allowed to flow therebetween. The plasma gas is thus transformed into the state of high-temperature plasma, and at the same time, a non-transferred plasma jet 13 is ejected through the opening formed at the forward end of the nozzle 12 toward a workpiece to be cut 14, thereby melting and cutting the workpiece 14. This non-transferred plasma cutting apparatus is advantageous in that the workpiece 14 can be cut by this apparatus whether it is formed of an electrically-conductive material or a non-conductive material. However, the apparatus presents a problem in that it incurs a greater loss of the nozzle and has a heat efficiency as low as approximately 20%. On the other hand, a typical transferred plasma cutting apparatus, as schematically shown in Fig. 7, is basically constructed in a manner similar to the non-transferred plasma cutting apparatus. That is, the transferred plasma cutting apparatus comprises an electrode 11 and a nozzle 12, all of the electrode 11, the nozzle 12 and the workpiece to be cut 14 being connected to the output terminal of the power supply source 16. A description will now be given of a cutting operation of the apparatuses constructed as described above. At the start of the operation, as illustrated in Fig. 6, arc discharge is effected between the electrode 11 and the nozzle 12 so that a non-transferred plasma jet 13 is ejected from the opening of the nozzle, thus generating a pilot arc. When this plasma jet 13 reaches the workpiece 14, electrical conductivity can be obtained between the electrode 11 within the torch and the workpiece 14. Subsequently, as illustrated in Fig. 7, a current flows between the electrode 11 and the workpiece 14, and simultaneously, the nozzle 12 and the power supply source 16 are disconnected so that the pilot arc can be extinguished, and a transferred plasma jet 15 can be formed. The starting operation of the transferred plasma arc is thus completed. Upon completion of the starting operation, the transferred plasma jet 15 starts to cut the workpiece 14. For achieving high performance of cutting, the plasma arc current applied during the cutting operation is set as the optimum value in consideration of various conditions, such as the shape of the plasma torch, the type of plasma gas, the type and the thickness of the workpiece to be cut, cutting speed and other conditions. The power supply source 16 is controlled so that this optimum current value can be maintained. In general, this plasma arc current is set to be constant without being changed until the cutting operation is completed. In contrast, when a cutting operation is performed on a workpiece having the shape of a small circular arc or an acute angle in cross section, it is required that the speed of a torch moving device be decreased for preventing vibration of the torch moving device and ensuring locus precision. For achieving these requirements, it is known that the following measures are taken. The plasma arc current is reduced in accordance with a decrease in the moving speed of the torch, thus preventing excessive heat from being transferred to the workpiece, thereby obtaining good cutting quality, in particular, the cutting quality exhibiting good appearance of corner portions (for example, refer to Japanese Utility Model Laid-Open No. 1-165171). Then, upon completion of the cutting operation, the transferred plasma arc current is interrupted, generally at a cutting finishing point. After the current interruption, the generation of heat required for performing the cutting operation is terminated, and the workpiece stops melting, thus completing the cutting operation. On the other hand, the following technique is known in contrast to the above-described cutting method. Upon completion of the cutting operation, the non-transferred pilot arc is ignited before the transferred plasma arc is extinguished so that the above-described pilot arc can be maintained at the cutting finishing point even after the transferred plasma arc has been extinguished, thus enabling speedy starting of a subsequent cutting operation (for example, refer to Japanese Patent Laid-Open No. 63-5875 and Japanese Patent Publication No. 4-9637). The typical transferred plasma cutting operation has thus been explained as described above. A higher heat efficiency of approximately 80% can be achieved by use of this method. Accordingly, this method is often employed in the metal processing field in which a workpiece is formed of a good conducting material. When a cutting operation is performed by the transferred plasma arc, for example, when a bore-cutting operation is performed on a metal workpiece to make a hole therein in the shape of a circle, ellipse, rectangle or the like, the cutting operation is generally performed according to a locus shown in Fig. 8. Referring to Fig. 8, reference numeral 24 denotes a cutting line; 21 indicates a cutting starting point; and 22 designates a cutting finishing point. The cutting operation is started by generating a transferred plasma arc at the cutting starting point 21, and then, the cutting operation is performed along the cutting line 24, followed by interrupting the transferred plasma arc current at the cutting finishing point 22, thus accomplishing the cutting operation. The reason that the cutting finishing point 22 is not set at a crossing point 23 on the cutting line 24 will be given below. If the cutting finishing point 22 is set at the crossing point 23, it is necessary to precisely provide timing for the movement of the plasma torch and the interruption of the arc current. If the arc current is interrupted too early, the workpiece is left partially uncut. In order to avoid this disadvantage, the cutting line 24 is extended inward toward the hole so that a reliable cutting operation can be ensured. However, in the above-described bore-cutting operation, when the size of a hole to be cut is small, a scrap workpiece 25 is frequently dropped without being held by a workpiece-supporting portion of the cutting apparatus. This causes a discharge point of the transferred plasma arc to remain on the inner surface of the hole formed in a product 26, as illustrated in Fig. 9. This arc discharge continues until the discharge arc is naturally extinguished due to the following movement of the plasma torch. More specifically, the plasma torch is moved to be away from the inner surface of the hole so as to cause an increase in the arc voltage, which makes it impossible to maintain discharging. Accordingly, until the arc discharge is extinguished, the inner surface of the hole is heated by the arc discharge, and also, it is likely that a double arc will be generated. This increases the degree of melting, burning or the like, occurring on the inner surface of the hole formed in the product 26, which further causes a problem of a deterioration in the cutting quality. The following measure is known to solve the above-described problem. Examination has been made on the arc voltage which is changed during the cutting operation depending on various conditions, such as the arc current value, gas pressure, cutting speed and other conditions. A set value of the arc voltage has then been determined. Thus, taking advantage of an increase in the arc voltage after the above-described scrap workpiece has been dropped, the plasma arc current is interrupted when the voltage exceeds the set value, thus shortening the duration of discharging applied to the workpiece. With this arrangement, a deterioration in the cutting quality can thus be effectively inhibited (for example, Japanese Patent Laid-Open No. 1-241379). However, in this method of taking advantage of the cutting arc voltage, it is necessary to predict a change in the voltage, such as an increase in the arc voltage depending on the cutting shape of, for example, a small circular arc, an acute angle or the like. Additionally, for preventing a stoppage of the cutting operation with respect to the arc voltage, it is necessary that a somewhat higher set value be determined. Because of these requirements, it will take longer time to interrupt the plasma arc current, whereby it is likely that the burning will be left on the cutting surface. DISCLOSURE OF INVENTION Accordingly, in order to overcome the above-described drawbacks inherent in the prior art, an object of the present invention is to provide a plasma cutting method and a NC plasma cutting apparatus in which an improvement can be made in the quality of the working surface in the vicinity of the cutting finishing point when a cutting operation is performed by use of a transferred plasma arc. According to a first aspect of the present invention, there is provided a plasma cutting method according to claim 1. Also, simultaneously with the switching operation to the non-transferred plasma arc, a moving speed of a plasma torch may be varied in accordance with a change from a transferred plasma arc current to a non-transferred plasma arc current. With this construction, even when a scrap workpiece is separated and dropped from a cut hole formed in a workpiece, a discharge point hardly remains in a resultant product. This can prevent an larger amount of heat than is necessary from being transferred to the product, thus obtaining a cutting surface of good quality. Further, the moving speed of the plasma torch may be varied, for example, the moving speed may be decreased to compensate for the lower heat efficiency of the non-transferred plasma arc, thus attaining suitable cutting capability. According to a second aspect of the present invention, there is provided a plasma cutting method according to claim 3. Also, simultaneously with reducing the transferred plasma arc current, a moving speed of a plasma torch may be varied in accordance with the reduction in the current. With this construction, even though a discharge point remains in a resultant product when a scrap workpiece is separated and dropped from a cut hole, the discharge point exhibits only a small plasma arc current, thus decreasing the heating value. Consequently, the product can be inhibited from melting, which further minimizes a deterioration in the quality of the cutting surface. In accordance with the aforementioned advantage, a decrease in the moving speed or the like may be realized to compensate for a reduction in the heating value due to a decrease in the plasma arc current, thereby attaining suitable cutting capability. According to a third aspect of the present invention, there is provided a plasma cutting method according to claim 5. Also, a function of interrupting the plasma arc current may instead be kept in the off state during the cutting operation and actuated in the above-described position. With this arrangement, it becomes possible to set a suitable voltage required for interrupting the arc current, without having the necessity of considering a change in the arc voltage caused by various factors during the cutting operation until reaching a position immediately before the cutting finishing point. This eliminates the necessity of setting an actual voltage higher than a suitable voltage, which conventionally has been necessary. Hence, it becomes possible to shorten the time required from dropping a scrap workpiece until the arc current is interrupted, thereby decreasing the heating value at a discharge point remaining in a resultant product, resulting in good quality of the cutting surface. With this method, it is possible to achieve the object more accurately, in comparison with a method of interrupting the arc current by detecting an increase in the arc voltage (for example, the above-discussed Japanese Patent Laid-Open No. 1-241379). Also, the plasma arc current interrupting function may be applied, in which case, operations and advantages similar to those obtained by the voltage detecting function can be attained. The present invention further provides a NC plasma cutting apparatus according to claims 7, 9 and 11. With this arrangement, it is possible to obtain a NC plasma cutting apparatus which makes changes to the proceeding cutting operation in a position immediately before the cutting finishing point and in the vicinity of a position in which cutting lines cross each other, such a position being instructed by the program. By use of this apparatus, a series of procedures of cutting processing can be automatically performed, and at the same time, the quality of cut products can also be improved. Further, in combination with the instruction means, the cutting processing can be performed to satisfy a demanded cutting quality without requiring complicated and troublesome operations. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a timing chart of a transferred plasma arc and a non-transferred plasma arc according to a first aspect of the present invention; Fig. 2 is a timing chart of a transferred plasma arc according to a second aspect of the present invention; Fig. 3 is a partial timing chart of a transferred plasma arc current and the moving speed of a plasma torch of another embodiment according to the second aspect of the present invention; Fig. 4 is a timing chart of a transferred plasma arc and an arc voltage detection device according to a third aspect of the present invention; Fig. 5 is a flow chart in which control is executed over a NC plasma cutting apparatus according to a fourth aspect of the present invention; Fig. 6 is a schematic view illustrative of the operation of a typical non-transferred plasma arc cutting apparatus according to a conventional art; Fig. 7 is a schematic view illustrative of the operation of a typical transferred plasma arc cutting apparatus according to a conventional art; Fig. 8 is a schematic view showing an example of a locus of a typical bore-cutting operation; and Fig. 9 shows that a bore-cutting operation is completed by use of a transferred plasma arc cutting apparatus according to a conventional art. BEST MODE FOR CARRYING OUT THE INVENTION A detailed description will now be given of a preferred embodiment of a plasma cutting method according to a first aspect of the present invention with reference to the accompanying drawings. Fig. 1 is a timing chart of this embodiment and shows a cutting starting point 1, an ignition point 2 of a non-transferred plasma arc, an extinguishing point 3 of a transferred plasma arc, a bore-cutting finishing point 4 and a cutting finishing point 5. This embodiment employs a NC plasma cutting apparatus of the type which performs a cutting operation based on a control program having normal cutting information and cutting conditions shown in Fig. 1 input therein. By use of this apparatus, a cutting operation is performed according to a locus of a typical bore-cutting operation on a metal workpiece. This embodiment will be described with reference to Figs. 1 and 8. In a manner similar to a typical cutting operation, the non-transferred plasma arc is ignited and extinguished so as to generate the transferred plasma arc in the position indicated by reference numeral 1 corresponding to the cutting starting point 21, thereby starting the bore-cutting operation. The cutting operation is performed along a cutting line 24 by the transferred plasma arc. Reference numerals 2 and 3 are set so that the cutting-proceeding position can be located in the vicinity of the crossing point 23 on the cutting line 24. More specifically, the cutting-proceeding position is more preferably located in a position immediately before the crossing point 23 when high precision is demanded for a cutting operation. On the other hand, the cutting-proceeding position is more preferably located in a position immediately after passing through the crossing point 23 when it is desired that the cutting speed take priority over precision. When the reference numerals 2 and 3 are set in a position immediately before the crossing point 23 the plasma torch, which has run generally one round along the cutting line 24, ignites the non-transferred plasma arc and at the same time, extinguishes the transferred plasma arc in a position immediately before the crossing point 23, thereby switching from the transferred plasma arc to the non-transferred plasma arc. After this switching operation, the plasma torch reaches the crossing point 23, which corresponds to reference numeral 4, on the cutting line 24 in a short time, thus accomplishing the bore-cutting operation. Accordingly, the cutting operation is performed by the non-transferred plasma arc during the interval between the positions indicated by reference numerals 3 and 4, i.e., the locus from immediately before the crossing point 23 to the crossing point 23. Subsequently, in a position indicated by reference numeral 5 corresponding to the cutting finishing point 22, an interruption signal is transmitted to a plasma supply source so that the non-transferred plasma arc is extinguished, thus accomplishing the cutting operation. It is not essential to provide a locus from the crossing point 23 at which the bore-cutting operation is actually finished to the cutting finishing point 22. Such an extra locus is, however, effective for ensuring a proper bore-cutting operation. In the non-transferred plasma arc, discharging occurs between the electrode and the nozzle of the plasma torch. Accordingly, it is operable even when there is no workpiece to be cut under the plasma torch, that is, when the scrap workpiece 25 is separated and dropped from the product 26. In the manner described above, the product 26 obtained by this embodiment does not leave any trace of a discharging point in the cut hole formed in the product 26 even when the scrap workpiece is dropped from the cut hole. It is thus possible to obtain a cutting surface of high quality. A detailed explanation will now be given of a preferred embodiment of the plasma cutting method according to a second aspect of the present invention with reference to the accompanying drawings. Fig. 2 is a timing chart of this embodiment. This embodiment employs a NC plasma cutting apparatus having cutting conditions shown in Fig. 2 input into a control program. By use of this apparatus, a bore-cutting operation is performed on a metal workpiece along a locus shown in Fig. 8 by virtue of a transferred plasma arc. This embodiment will be explained with reference to Figs. 2 and 8. First of all, the transferred plasma arc is generated in a position indicated by reference numeral 1 by use of the non-transferred plasma arc (not shown), thereby starting the bore-cutting operation from the cutting starting point 21. The operation is performed along the cutting line 24. Reference numerals 6 and 7 are set so that the cutting-proceeding position can be located in the vicinity of the crossing point 23, and more preferably, in a position immediately before the crossing point 23. During the interval between reference numerals 6 and 7, the transferred plasma arc current at a point indicated by 6 is reduced to such a degree as to reach a level of the current indicated by reference numeral 8. In the state of the plasma arc with the reduced current, in a short time the plasma torch reaches the crossing point 23, which is the bore-cutting finishing point, corresponding to reference numeral 4, thus accomplishing the bore-cutting operation. The reason that the transferred plasma arc is extinguished from the bore-cutting finishing point indicated by 4 onward will be given below. In the transferred plasma arc in which arc discharging occurs between the electrode within the plasma torch and a workpiece to be cut, there is an increase in the voltage when a scrap workpiece is dropped, whereby the transferred plasma arc is unable to be maintained. It is needless to say, however, that the input timing chart may be modified in such a manner that the transferred plasma arc is not extinguished in the position indicated by 4, and instead, the cutting operation is continued at a level of the current indicated by 8, and the transferred plasma arc is extinguished in the position indicated by 5 corresponding to the cutting finishing point 22, thus accomplishing the cutting operation. In such a case, the transferred plasma arc is often naturally extinguished before reaching the cutting finishing point 22 due to an increase in the arc voltage. In the manner described above, the current of the transferred plasma arc is reduced immediately before the crossing position 23 on the cutting line 24 so that only a small amount of current flows at a discharge point which remains in the product even when the scrap workpiece is separated and dropped from a hole formed by the cutting operation. Hence, only a very small trace of discharging is left, and thus, it is possible to maintain the high quality of the cutting surface of the product. In contrast to the cutting operation performed by reducing the transferred plasma arc current in the above-described embodiment, a description will further be given of an embodiment in which the moving speed of the plasma torch is varied in accordance with the reduction in the transferred plasma arc current. Fig. 3 shows one example of a timing chart of the time interval between reference numerals 6 and 7 shown in Fig. 2 illustrating the previous embodiment. In this embodiment, the transferred plasma arc current is reduced, and simultaneously, the moving speed of the plasma torch is decreased in accordance with this change in the current. With this arrangement, even when the cutting capability is changed due to a change in the plasma arc current, suitable cutting conditions can be obtained by varying the moving speed of the torch, and more preferably, by decreasing the moving speed, thus obtaining better quality of the cutting surface. In the embodiment according to the first aspect of the present invention, as well as this embodiment, advantages similar to those obtained in this embodiment can be realized by the following process, which is more preferable. That is, the transferred plasma arc is switched to the non-transferred plasma arc, and simultaneously, the moving speed of the plasma torch is changed, and more specifically, the moving speed is decreased, in accordance with the switching from the transferred plasma arc current to the non-transferred plasma arc current. A detailed description will now be given of a preferred embodiment of a plasma cutting method according to a third aspect of the present invention with reference to the accompanying drawings. Fig. 4 is a timing chart of this embodiment. This embodiment employs a NC plasma cutting apparatus provided with an arc voltage detection device for detecting a voltage of a transferred plasma arc and interrupting a plasma arc current when the detected voltage exceeds a set value (for example, a device disclosed in the above-described Japanese Patent Laid-Open No. 1-241379). By use of this apparatus, a bore-cutting operation is performed on a metal workpiece along the locus shown in Fig. 8. This embodiment will be explained with reference to Figs. 4 and 8. First of all, a transferred plasma arc is generated in a position indicated by reference numeral 1 by a non-transferred plasma arc, thereby starting a bore-cutting operation from the cutting starting point 21. The cutting operation is performed along the cutting line 24. Since the cutting operation was started, the voltage detecting function of the arc voltage detection device has been turned off, that is, is inoperable. Subsequently, the voltage detecting function of the arc voltage detection device is actuated in a position indicated by reference numeral 9, that is, in a position immediately before the crossing point 23 of the cutting lines. With this arrangement, it becomes possible to set a suitable voltage required for interrupting the arc current to complete a cutting operation, regardless of a change in the arc voltage caused by various factors during the cutting operation, which further eliminates the necessity of setting an actual voltage higher than a suitable voltage, which conventionally has been necessary. This can shorten the time required for dropping the scrap workpiece 25 until the arc current is interrupted, thereby minimizing any trace of a discharge point left in a resultant product. It should be noted that the timing chart may be modified in a manner described below. The transferred plasma arc may not be extinguished in a position indicated by reference numeral 4, which is the bore-cutting finishing point corresponding to the crossing point 23 on the cutting line 24, and instead, the cutting operation may be continued while the transferred plasma arc is kept in the ignition state. Then, the plasma arc may be extinguished in a position indicated by reference numeral 5 corresponding to the cutting finishing point 22, thus accomplishing the cutting operation. Additionally, in place of the voltage detecting function provided with the arc voltage detection device, a plasma arc current interrupting function provided with the arc voltage detection device may be employed. In such a case, operations and advantages similar to those obtained by the foregoing embodiments can be attained. A detailed explanation will now be given of a preferred embodiment of a NC plasma cutting apparatus according to a fourth aspect of the present invention with reference to the accompanying drawings. Fig. 5 is a schematic flow chart illustrative of one embodiment in which control is executed over a NC plasma cutting apparatus, and more specifically, a flow chart having a program in which a transferred plasma arc is switched to a non-transferred plasma arc in a position immediately before a cutting finishing point and in a position in the vicinity of a crossing point of cutting lines. This apparatus is similar to typical NC plasma cutting apparatuses in that a cutting operation is performed along a cutting shape from a cutting starting point based on the input program. In step 51, data concerning a cutting shape, cutting conditions and a crossing point in a position immediately before the cutting finishing point is first read. The cutting conditions are set in step 52, and a cutting operation is started in step 53. Subsequently, in step 54, it is determined whether a position in which the cutting operation is currently proceeding matches the data on the position in the vicinity of the crossing point of the cutting lines and the position immediately before the cutting finishing point. The data on the crossing point on the cutting lines may be represented by coordinates by way of example. Also, a determination of whether a position in which the cutting operation is currently proceeding is near the above-described coordinates may be made by means such as determining whether such a position is located within a region at a predetermined distance away from the coordinates. When it is determined that the position in which the cutting operation is currently proceeding is located in the vicinity of the crossing point, the flow proceeds to step 55 in which the transferred plasma arc is switched to the non-transferred plasma arc. The cutting operation is started again based on the cutting shape (step 56). When the cutting finishing point is reached, the power supply source is turned off in step 57, thus accomplishing the cutting operation. On the other hand, when it is determined in step 54 that the currently-cutting position is not located in the vicinity of the crossing point of the cutting lines, the flow proceeds to step 58 in which the cutting operation is continued based on the cutting shape, and then, the flow returns to step 54. According to the process discussed above, even when a scrap workpiece is separated and dropped from the cut hole, a trace of a discharge point is not left on the cut hole formed in a resultant product, thereby obtaining a cutting surface of good quality. Although the switching operation from the transferred plasma arc to the non-transferred plasma arc is performed in step 55 in this embodiment, the foregoing plasma cutting method according to the present invention may instead be input in step 55. For example, the below-described instructions are input. Simultaneously with the switching operation from the transferred plasma arc to the non-transferred plasma arc, the moving speed of the plasma torch is varied in accordance with a change from the transferred plasma arc current to the non-transferred plasma arc current. The transferred plasma arc current is reduced. Simultaneously with this instruction for reducing the current, the moving speed of the plasma torch in accordance with the reduction in the current is varied. The voltage detecting function is kept in the off state during the cutting operation since the cutting operation is started (step 51), and in a position immediately before the cutting finishing point and in the vicinity of the crossing point of the cutting lines, the above-mentioned function is actuated. The plasma arc current interrupting function is kept in the off state during the cutting operation since the cutting operation is started (step 51), and in a position immediately before the cutting finishing point and in the vicinity of the crossing point of the cutting lines, the above mentioned function is actuated. The above-described instructions may be input so as to obtain a cutting surface of good quality. The instructions may be combined with each other and input as required. With the NC plasma cutting apparatus by application of the above-discussed program, automatic cutting processing to satisfy demands of various types of cutting qualities and cutting speeds can be achieved, thus eliminating the need for complicated and troublesome operations and improving productive efficiency. INDUSTRIAL APPLICABILITY The present invention is useful as a plasma cutting apparatus and a plasma cutting method in which an improvement can be made in the quality of a bore-cut product by use of a transferred plasma arc, and also, the productive efficiency can be enhanced.

Claims: A plasma cutting method for cutting a hole in a workpiece (26) by generating a transferred plasma arc between an electrode and the workpiece and cutting the workpiece along a cutting line which extends from a cutting starting point (21) to a cutting finishing point (22) such that said cutting line crosses itself at a crossover position (23), characterized in that said transferred plasma arc is switched to a non-transferred plasma arc in a position immediately before the cutting finishing point (22) and in the vicinity of the crossover position (23). A plasma cutting method according to claim 1, characterized in that when said transferred plasma arc is switched to said non-transferred plasma arc, simultaneously, a moving speed of a plasma torch is varied in accordance with a change from a transferred plasma arc current to a non-transferred plasma arc current. A plasma cutting method for cutting a hole in a workpiece (26) by generating a transferred plasma arc between an electrode and the workpiece and cutting the workpiece along a cutting line which extends from a cutting starting point (21) to a cutting finishing point (22) such that said cutting line crosses itself at a crossover position (23), characterized in that a transferred plasma arc current is reduced in a position immediately before a cutting finishing point (22) and in the vicinity of the crossover position (23). A plasma cutting method according to claim 3, characterized in that when said transferred plasma arc current is reduced, simultaneously, a moving speed of a plasma torch is varied in accordance with the reduction in the current. A plasma cutting method for cutting a hole in a workpiece (26) by generating a transferred plasma arc between an electrode and the workpiece and cutting the workpiece along a cutting line which extends from a cutting starting point (21) to a cutting finishing point (22) such that said cutting line crosses itself at a crossover position (23), comprising the steps of: generating a transferred plasma arc between an electrode and a workpiece to be cut; detecting a voltage across said electrode and said workpiece or a voltage across a nozzle and said workpiece when said workpiece is cut; and interrupting a plasma arc current when the detected voltage exceeds a set value, said method being characterized in that a function of detecting said voltage is kept in the off state during a cutting operation, and said function is actuated in a position immediately before a cutting finishing point (22) and in the vicinity of the crossover position (23). A plasma cutting method according to claim 5, characterized in that a function of interrupting said plasma arc current is kept in the off state during the cutting operation and actuated in the position immediately before the cutting finishing point (22) and in the vicinity of the crossover position (23). A NC plasma cutting apparatus for cutting a workpiece based on a control program having cutting information, such as a cutting shape, a cutting procedure, cutting conditions, and the like input therein, said apparatus comprising: means for inputting in advance in said control program data concerning a position (23) immediately before a cutting finishing point (22) and in which cutting lines cross each other; means for detecting during a cutting operation that said position (23) which has been input into said control program has nearly been reached; and means for giving an instruction, based on the result of said detection, for switching from a transferred plasma arc to a non-transferred plasma arc. A NC plasma cutting apparatus according to claim 7, characterized in that said instruction means is means for giving said switching instruction and also for giving an instruction for varying a moving speed of a plasma torch in accordance with a change from a transferred plasma arc current to a non-transferred plasma arc current. A NC plasma cutting apparatus for cutting a workpiece based on a control program having cutting information, such as a cutting shape, a cutting procedure, cutting conditions, and the like input therein, said apparatus comprising: means for inputting in advance in said control program data concerning a position (23) immediately before a cutting finishing point (22) and in which cutting lines cross each other; means for detecting during a cutting operation that said position (23) which has been input into said control program has nearly been reached; and means for giving an instruction, based on the result of said detection, for reducing a transferred plasma arc current. A NC plasma cutting apparatus according to claim 9, characterized in that said instruction means is means for giving said current-reducing instruction and also for giving an instruction for varying a moving speed of a plasma torch in accordance with said current reduction. A NC plasma cutting apparatus for cutting a workpiece based on a control program having cutting information, such as a cutting shape, a cutting procedure, cutting conditions, and the like input therein, said apparatus comprising: means for inputting in advance in said control program data concerning a position (23) immediately before a cutting finishing point (22) and in which cutting lines cross each other; means for detecting during a cutting operation that said position (23) which has been input into said control program has nearly been reached; and means for giving an instruction, based on the result of said detection, for allowing a voltage detecting function to be kept in the off state during the cutting operation and for actuating said function in the position immediately before the cutting finishing point (22) and in the vicinity of the position (23) in which the cutting lines cross each other. A NC plasma cutting apparatus according to claim 11, characterized in that said voltage detecting function is a plasma arc current interrupting function.

Inventors: HASHIZUME YOSHIHIKO, NAKAU YUTAKA, SHINTANI TOSHIYA, HASHIZUME, YOSHIHIKO, NAKAU, YUTAKA, SHINTANI, TOSHIYA, HASHIZUME, Yoshihiko, Komatsu Bldg. 8F, NAKAU, Yutaka, Komatsu Head Office, SHINTANI, Toshiya, Komatsu Res. Lab., 1200, Manda
Applicants: KOMATSU MFG CO LTD, KABUSHIKI KAISHA KOMATSU SEISAKUSHO
Application year: 1994
Main classification: B23K10
IPC codes: B23K10